Page 125 - Geothermal Energy Systems Exploration, Development, and Utilization

2.5 Geochemistry 101
almost impossible to find a solution to corrosion and scaling problems except on a
specific basis for particular sites.
The chemistry from different fields can vary substantially. Higher temperature
resources with the higher water ratios have increased levels of silica that cause
tremendous scaling and deposit problems. By contrast, dry steam fields do not
experience the silica problems, but instead have aggressive corrosion problems as-
sociated with hydrogen chloride and hydrogen sulfide attack. Still other geothermal
fields are hit with a double misfortune and encounter both scaling and corrosion
problems at thesametime(Figure2.20).
Corrosion attacks occur in many geothermal operations and these result in severe
equipment damage (Lichti and Wilson, 1999). Corrosion may be general, a sort
of ‘‘rusting’’ that affects all metal surfaces in geothermal equipment, or localized.
There are multiple mechanisms, for example, stress corrosion and sulfide stress
cracking, of corrosion attack contributing to the failure of pipe and equipment in
these systems (Corsi, 1986). The first is due to stress coupled with the existence
of chloride ions in the environment and the second is caused by the presence
of hydrogen sulfide in an aqueous phase. The thermal and hydraulic machines
require materials that can withstand stress as well as corrosion erosion resulting

Figure 2.20 Wellheadsubmittedtocorrosion andscaling. (PhotoF.-D. Vuataz).